What is a control variable?

• A. A variable changes randomly during an experiment.
• B. The independent variable being tested.
• C. The dependent variable measured at the end.
• D. A variable kept constant to ensure observed effects are due to manipulation of the independent variable.

Answer: (D)

A control variable is something kept constant in an experiment so that any observed effect can be linked to the factor you are purposely changing. By holding other conditions steady, you can tell whether changes in the outcome are due to the independent variable, not some other factor. For example, in a plant growth study you would keep water, soil, pot size, and temperature the same and only vary the amount of light. Then you measure growth as the dependent variable. The other ideas describe different parts of an experiment: a variable that changes randomly refers to random error, the independent variable is what you change, and the dependent variable is what you measure. Keeping control variables constant helps show cause and effect.